The 4.3 I think came out in about 91 and was found in the Chev s10. Your 3.1 is a GM engine also found in the Bretta, Corsica and camero.
They GM 2.8, 3.1 and 3.4 are all the same basic engine but are just bored out a bit.
But I think the 4.3 Trottle body will fit on the 3.1. As for gains, may 10 hp?
The filter you may see 5-7 HP and muffler another 5-7.
Not much. I would just stick with the new filter and muffler and forget the TB (too much effort and cost for a couple of HP).
The 93-97 rodeos are getting pretty cheap so it might be worth your time to keep an eye out for a cheap Rodeo and sell yours. If that is an option.